Conventionally, with the aim of reduction in weight of a steel sheet, an attempt to increase strength of a steel sheet has been promoted. Generally, the increase in strength of a steel sheet causes deterioration of formability such as bore expandability. Therefore, it is important how a steel sheet excellent in balance between tensile strength and bore expandability is obtained.
For example, in Patent Literature 1, there has been disclosed a technique aiming to obtain a steel sheet excellent in balance between tensile strength and bore expandability by optimizing a fraction of microstructure such as ferrite and bainite in steel and precipitates in a ferrite structure. In Patent Literature 1, it has been described that the tensile strength of 780 MPa or more and a bore expansion ratio of 60% or more are obtained.
However, in recent years, a steel sheet more excellent in the balance between the tensile strength and the bore expandability has been required. For example, a steel sheet used for an underbody member of an automobile or the like has been required to have the tensile strength of 780 MPa or more and the bore expansion ratio of 70% or more.
Further, the bore expansion ratio is likely to vary relatively. Therefore, for improving the bore expandability, it is important to decrease not only an average λave of the bore expansion ratio but also a standard deviation σ of the bore expansion ratio being an index indicating the variations. Then, in the steel sheet used for an underbody member of an automobile or the like as described above, the average λave of the bore expansion ratio has been required to be 80% or more, and the standard deviation σ has been required to be 15% or less and has been further required to be 10% or less.
However, conventionally, it has been difficult to satisfy these requirements.
Further, in a case when an automobile drives over a curb or the like to thereby apply a strong impact load to its underbody part, ductile fracture is likely to occur starting from a punched face of the underbody part. Particularly, as a steel sheet has higher strength, its notch sensitivity is higher, and thus the fracture from a punched edge face is more strongly concerned. Thus, as a steel sheet has higher strength, it is important to prevent the ductile fracture as described above. Therefore, in the steel sheet used as a structure member such as the underbody part as above, it is also important to improve the fracture property.